1. Field of the Invention
The present invention relates to a multiband antenna and an electronic device.
2. Description of Related Art
Mobile devices having wireless communication functions, such as a handy terminal and a personal digital assistant (PDA), have conventionally been known. A multiband antenna having a plurality of resonance frequencies has been known as one of wireless communication antennas to be mounted on such mobile devices.
Moreover, a parallel two-line antenna having two resonance frequencies has been known as one of the multiband antennas (see, for example, Japanese Patent Application Laid-Open Publication No. 2009-124355). The parallel two-line antenna can have two resonance frequencies by closely arranging two radiative conductors having the same lengths in a folded-back shape.
Moreover, a configuration using a monopole antenna, the structure of which is simpler than that of the parallel two-line antenna, as one of the multiband antennas has been known. The monopole antenna has the configuration of using one side of two antenna elements of a half-wave dipole antenna as the ground thereof.
FIG. 26 shows the configuration of a conventional monopole antenna 80. As shown in FIG. 26, the monopole antenna 80 includes an antenna element 81 and a ground section 82. A feeding point P is arranged between the antenna element 81 and the ground section 82. The antenna element 81 has a length of (¼)λ0 (λ0: the wavelength of a predetermined frequency), and resonates at a frequency f0 corresponding to the wavelength λ0 and a frequency 3f0 that is three times as large as the frequency f0. The monopole antenna consequently functions as a multiband antenna resonating at the two frequencies f0 and 3f0.
Moreover, a monopole antenna having a folding-back structure of folding back the tip of an antenna element has been known (see, for example, Japanese Patent Application Laid-Open Publication No. 2001-223519). The monopole antenna of the folding-back structure resonates at a frequency higher than the frequency f0 and a frequency lower than the frequency 3f0 owing to an interaction of the folded-back part.
Moreover, a wireless LAN system has been known as a wireless communication system of a mobile device. IEEE 802.11b/g for a frequency band of a 2.4 GHz band and IEEE 802.11a for a frequency band of a 5 GHz band exist as the standards of wireless LAN systems.
The conventional monopole antenna of the folding-back structure however was not capable of being applied to a multiband antenna resonating in two frequency bands of wireless LAN communication. To put it concretely, when the length of the antenna element of a monopole antenna having the folding-back structure was adjusted in order that the resonance frequency corresponding to the frequency f0 may be adjusted to 2.45 [GHz], the resonance frequency thereof corresponding to the frequency 3f0 was approximately 7.38 [GHz].
The necessary frequency band of the 2.4 GHz band of the IEEE 802.11b/g is a band of from 2.4 to 2.5 [GHz]. Moreover, the necessary frequency band of the 5 GHz band of the IEEE 802.11a is a band of from 5.18 to 5.7 [GHz]. Consequently, even if the shifts of frequencies when a monopole antenna of the folding-back structure was incorporated in a housing were considered, no resonance frequencies satisfying the two necessary frequency bands of wireless LAN communication were capable of being obtained.